The present disclosure relates to a developing device for developing an electrostatic latent image formed by electrophotography and an image forming apparatus provided with the developing device.
A developing device employing a touch-down type (also referred to as a hybrid type) is known. In the developing device of this type, a two-component developer including at least a carrier and toner is supported on a surface of a developer supporting rotator to form a magnetic brush on the surface by the carrier. In this type, the toner supplied from the magnetic brush to a toner supporting rotator forms a toner layer on a surface of the toner supporting rotator. The developing device causes the toner to fly from the toner layer thus formed to the image carrier so as to develop the electrostatic latent image on a surface of the image carrier into a toner image.
In the developing device of the touch-down type, a so-called development history may easily occur. The development history is a difference in developing performance at a time of subsequent rotation of the toner supporting rotator between a part where the toner is used for development and a part where the toner is not used to remain on the surface of the toner supporting rotator, of the toner of the toner layer formed on the surface of the toner supporting rotator. If the developing history occurs, the toner layer formed on the surface of the toner supporting rotator turns uneven. Accordingly, it is likely that the density of an image decreases due to a change in toner characteristics (particle size, triboelectric charge and the like), and the image quality degrades, the toner scatters, and developing defect such as developing ghost occurs due to long-neglected undeveloped toner.
In order to suppress an occurrence of the abovementioned developing defect, it is necessary to collect (strip) the undeveloped toner on the surface of the toner supporting rotator toward the developer supporting rotator between two successive operations of development and at a time without transfer of an image. As a method for stripping the undeveloped toner, a developing device has been known, in which a first time period is set shorter than a second time period. During the first time period, an alternating component of a bias voltage is applied to cause the undeveloped toner to migrate from the toner supporting rotator to the developer supporting rotator (strip and collection). During the second time period, an alternating component of a bias voltage is applied to cause the toner to migrate from the developer supporting rotator to the toner supporting rotator.
In addition, a developing device has been known, in which unevenness in a thickness of a toner layer between a part consuming the toner and a part without consuming the toner on the toner supporting rotator is configured to decrease. In order to decrease the unevenness, a magnetic member is disposed inside the developer supporting rotator, such that a half magnitude region having magnetic flux density of one half of a peak value of the magnetic member is set over a broad range where a gap is several times greater than the narrowest gap between the developer supporting rotator and the toner supporting rotator. In this manner, a time period during which the two-component developer is in contact with the surface of the toner supporting rotator increases and sufficiently supplies toner to the part consuming toner.
In the above-described developing device, density of the magnetic brush (mg/cm2) and an orientation of an opposing magnetic brush at a time of the closest approach of the toner supporting rotator are important factors to perform stripping of the undeveloped toner on the surface of the toner supporting rotator. In other words, mechanical rubbing due to the density of the magnetic brush and the intensity of electric field between the toner supporting rotator and the developer supporting rotator (alternating component of a bias voltage on a stripping side) correlating to the orientation of the magnetic brush affect the stripping of the toner. In view of what has been described above, both of the above-described developing devices exert an effect of stripping the undeveloped toner in a developing region (image forming region) on the surface of the toner supporting rotator.
However, the effect of stripping the undeveloped and adhered toner is insufficient at longitudinal end portions of the toner supporting rotator, for the following reasons. Since the magnetic brush does not face the longitudinal end portions of the toner supporting rotator, a sneak magnetic field is formed at the longitudinal end portions of the developer supporting rotator. Since the direction of the sneak magnetic field is not in line with a normal vector connecting centers of the developer supporting rotator and the toner supporting rotator and is inclined relative to the normal line. Accordingly, the effect of stripping the undeveloped toner, for which the density and orientation of the magnetic brush are important factors, is not sufficiently exerted. As a result, the undeveloped toner adheres to (remains on) the surface of the longitudinal end portions of the toner supporting rotator and the undeveloped and adhered toner is electrically charged after successive operations. This may cause a problem of fogging at both end portions of a toner image on the surface of the image carrier.